In the steam system of a PFBC power plant an optimum dimensioning of feedwater preheaters, economizers, evaporators and reheaters entails special problems under certain operating conditions, for example at low or very low load. A PFBC power plant also comprises a gas cycle, in which combustion gases from the combustor of the plant drive a gas turbine. Upon, for example, start-up of the plant or upon a gas turbine trip or upon an emergency stop of the combustor in the plant, the steam flow through the superheater and/or the reheater may become insufficient. As a result, too high a temperature is attained in the boiler tubes of the steam heater and/or reheater. Consequently, these boiler tubes may become damaged, because the superheater and reheater tubes do not receive sufficient cooling under the above-mentioned operating conditions.
During the bed heating phase and when changing to coal firing upon start-up of a PFBC power plant, as well as in case of a gas turbine or a steam turbine trip in the plant, the cooling of the surfaces of the superheater and reheater tubes must be ensured. The necessary steam flow through these tubes for cooling of the tube surfaces must be maintained. This is particularly necessary for the tube bundle of the reheater since, in a PFBC boiler where the tube bundles of the superheater and the reheater in the vertical direction are uniformly distributed in the bed, these tube bundles are heated simultaneously and to the same extent. Contrary to a conventional boiler, in the PFBC boiler a constant temperature of the superheated steam can be maintained from full load approximately down to Benson load, without having to take measures, such as injection of abnormal quantities of water, flue gas recirculation or similar known methods. This is an advantage from the point of view of efficiency and transients and is achieved in a PFBC boiler by the selection of a suitable tube bundle geometry for the reheater. However, this also means that the lowest tube surfaces of the superheater and reheater tube bundles are disposed at approximately the same level in the bed. Upon start-up of the boiler, these tube bundles will thus be heated simultaneously, that is, they both need steam for cooling at the same time. With conventional steam system solutions and especially upon cold-start of the plant, the steam flow reaches the tube bundle of the reheater late and is not of sufficient magnitude. This is due to the fact that steam disappears through open valves, part of the steam is condensed upon heating of cold surfaces in the live steam and reheater conduits between the boiler and the steam turbine and, finally, part of the steam is used to build up the pressure in the volumes of these conduits.
It would be desirable to give priority to the steam production in both the superheater and the reheater from start up to operation at Benson load and that these heating surfaces are ensured cooling also in the event of a steam turbine trip or a sudden load reduction without the boiler having to be stopped.